Positive displacement pumps are often employed at oilfields for large high pressure applications involved in hydrocarbon recovery efforts. A positive displacement pump may include a plunger driven by a crankshaft toward and away from a chamber in order to dramatically effect a high or low pressure on the chamber. This makes it a good choice for high pressure applications. Indeed, where fluid pressure exceeding a few thousand pounds per square inch (PSI) is to be generated, a positive displacement pump is generally employed.
Positive displacement pumps may be configured of fairly large sizes and employed in a variety of large scale oilfield operations such as drilling, cementing, coil tubing, water jet cutting, or hydraulic fracturing of underground rock. Hydraulic fracturing of underground rock, for example, often takes place at pressures of 10,000 to 15,000 PSI or more to direct a solids containing fluid through a well to release oil and gas from rock pores for extraction. Such pressures and large scale applications are readily satisfied by positive displacement pumps.
As noted, a positive displacement pump includes a plunger driven toward and away from a pressurizable chamber in order to achieve pumping of a solids containing fluid. More particularly, as the plunger is driven away from the chamber, pressure therein reduces allowing a discharge valve of the chamber to close. The chamber is thus sealed off from the external environment while the plunger remains in communication with the chamber. As such, the plunger continues its retreat away from the chamber generating a lowered pressure with respect to suction therein. Eventually, this lowered pressure will reach a level sufficient to open a suction valve of the pump in order to allow an influx of fluid into the chamber. Subsequently, the plunger may be driven toward the chamber to once again effect a high pressure therein. Thus, the suction valve may be closed, the discharge valve re-opened, and fluid expelled from the chamber as indicated above.
The actuation of the suction and discharge valves is achieved primarily through reliance on pressure conditions generated within the chamber. That is, the amount of pressure required to open or close each valve is a function of the physical characteristics of the valve along with a spring employed to hold the valve in a naturally closed position relative to the chamber. Unfortunately, this results in a lack of direct control over valve actuation and leaves an inherent inefficiency in operation of the valves. For example, opening of a valve requires generation of enough of a pressure change so as to overcome the weight of the valve and nature of its spring. This is of particular note regarding the suction valve where, rather than opening immediately upon closure of the discharge valve, a lowered pressure sufficient to overcome the weight and nature of the suction valve and its spring must first be generated within the chamber (i.e. net positive suction head (NPSH)). This time delay in opening of the suction valve is an inherent inefficiency in operation of the pump. Indeed, for a standard positive displacement pump employed at an oilfield, a pressure of between about 10 PSI and about 30 PSI may be required within the chamber before the suction valve is opened.
Reliance solely upon internal chamber pressure to actuate valves results in an inherent inefficiency and a lack of direct control as indicated above. One such concern is the fact that this manner of valve actuation often leaves the pump itself susceptible to significant damage as a result of cavitation and ‘water hammering’. That is, as the plunger moves away from the chamber decreasing pressure therein, the inherent delay in opening of the suction valve may lead to the cavitation and subsequent water hammering as described below.
During the delay in opening of the suction valve, and in conjunction with the generation of lowered pressure in the chamber, the fluid may undergo a degree of cavitation. That is, pockets of vapor may form within the fluid and it may begin to vaporize in the face of the lowered pressure. Formation of vapor in this manner may be followed by rapid compression of the vapor back into liquid as the plunger once again advances toward the chamber. This rapid compression of the liquid is accompanied by a significant amount of heat and may also result in transmitting a degree of shock through the pump, referred to as water hammering. All in all, a significant amount of pump damage may naturally occur based on the pressure actuated design of a conventional positive displacement pump.
In order to address pump damage resulting from cavitation and water hammering, techniques are often employed in which acoustic data generated by the pump is analyzed during its operation. However, reliance on the detection of acoustic data in order to address pump damage fails to substantially avoid the development of pump damage from cavitation and water hammering in the first place. Furthermore, it is not uncommon for the damaged pump to be employed in conjunction with an array of additional pumps at an oilfield. Thus, the damage may see its effects at neighboring pumps, for example, by placing added strain on these pumps or by translation of the damaging water hammering effects to these pumps. Indeed, cascading pump failure, from pump to pump to pump, is not an uncommon event where a significant amount of cavitation and/or water hammering is found.
It is desirable to improve the operation and reliability of pumps, such as those comprising at least one active valve controlling for suction/discharge. It is desirable to improve the volumetric efficiency of pumps, reduce the likelihood of valve wear, and improved control of the pump operation including, but not limited to, the ability to control the volume output of the pump.